In an optical communication network, an optical data stream is typically produced by modulating an optical carrier based upon a data signal. Multiple optical data streams having different wavelengths are often multiplexed onto a single optical fiber using a technique known as Wavelength Division Multiplexing (WDM). WDM allows a single optical fiber to carry multiple optical data streams.
At various nodes in the optical communication network, it is often necessary or desirable to re-route optical data streams among and between various fibers. For example, certain optical data streams from an incoming fiber may be passed through to an outgoing fiber, while other optical data streams from the incoming fiber are not passed through to the outgoing fiber. For convenience, optical data streams that are pass through from the incoming fiber to the outgoing fiber are referred to hereinafter as “passed” optical data streams, while optical data streams that are not passed through from the incoming fiber to the outgoing fiber are referred to hereinafter as “dropped” optical data streams. Furthermore, new optical data streams may be inserted onto the outgoing fiber. For convenience, such new optical data streams are referred to hereinafter as “added” optical data streams. For convenience, an apparatus that performs such pass, drop, and add functions is often referred to as an optical add/drop multiplexer.
A typical optical add/drop multiplexer includes a demultiplexer for demultiplexing optical data streams from an incoming fiber, an add/drop fabric for performing the add/drop functions, and a multiplexer for multiplexing optical data streams onto an outgoing fiber. Specifically, optical data streams demultiplexed from the incoming fiber are fed as inputs to the add/drop fabric, as are any optical data streams to be added. At each wavelength, the add/drop fabric may be configured to pass the demultiplexed optical data stream, drop the demultiplexed data stream and add a new optical data stream at the same wavelength, or drop the demultiplexed data stream without adding a new optical data stream at the same wavelength. Thus, the added data streams are limited to the wavelengths of the dropped data streams. The multiplexer receives passed and/or added optical data streams from the add/drop fabric, and multiplexes the passed and/or added optical data streams onto the outgoing fiber.
The multiplexer portion of the optical add/drop multiplexer typically includes filter logic that prevents any out-of-band optical data stream from interfering with another optical data stream. Specifically, the filter logic monitors the wavelength of each optical data stream. If the wavelength of a particular optical data stream shifts outside of a predetermined range, the filter logic prevents that optical data stream from being multiplexed onto the outgoing fiber. The wavelength of an optical data stream can shift, for example, due to certain laser failures (e.g., the control voltage applied to the laser). While this filter logic prevents interference between optical data streams, it also adds cost and complexity to the optical add/drop multiplexer.